The invention relates to a method of routing traffic in satellite communications systems. The invention also relates to access units and gateways for such systems.
A communications system such as the Internet comprises a multitude of networks which are inter-connected by gateways. For instance, a local area network, which could comprise all the workstations in one company, may be connected by a gateway to a wide area network. Information is distributed around the Internet from sources, for example, hosts, to destinations, for example workstations, and vice versa, in the form of datagrams. The datagrams include data, which is information to be transmitted from sources to destinations, and destination information, that is, an address. The most common form of gateways are three layer gateways (or routers) which route datagrams, having assessed the destination information, in accordance with their knowledge of the organisation of the network to which they are connected.
Most networks have an appointed default gateway through which all traffic is routed. There may be other gateways on the network, and traffic destined for certain locations within the network may be more appropriately routed through one of these other gateways. When the default gateway receives a datagram for which it considers, by assessment of the destination information, that another one of the gateways may be more appropriate, it redirects the datagram.
When communications between networks are performed via satellite, and access units route traffic to and from a source to the satellite, redirecting a datagram may involve sending it over the satellite system""s links, which consumes satellite resources and introduces a delay into the transmission of the datagram. To overcome these problems, when a gateway receives from a satellite a datagram for which it considers that an alternative gateway unit may be more appropriate, it sends an internet control message protocol (ICMP) redirect message to the source of the datagram, suggesting that the datagram be sent directly to the alternative gateway, thereby cutting out the unnecessary additional step for all future transmissions.
In a system having a satellite link between networks, there may be a large number of sources, which means that there are potentially a large number of ICMP redirect messages passing around the system at any one time, which in turn may lead to congestion. Alternatively, each access unit may operate with routing protocols, but this may introduce huge complexity and does not reduce the amount of information carried over the satellite links.
In a local area network, sending a redirect message for future reference to all sources is not considered feasible because it entails extra processsing on the part of each source.
The object of the invention is to provide a method of efficiently routing transmissions in a satellite communications system which minimises the amount of control information in circulation.
According to a first aspect of the present invention there is provided a method of routing transmissions in a satellite communications system comprising a plurality of access units communicating with a satellite on an up-link and a broadcast downlink and a plurality of gateways to and from a network, each gateway receiving from the satellite transmissions communicated thereto by the access units and intended for destinations in the network, the method including a gateway concluding that a transmission received should follow an alternative route via one of the other gateways, sending an alternative route message to the satellite which forwards the message using the broadcast downlink to a plurality of access units in the system.
The invention takes advantage of the inherent broadcast capability of the satellite by disseminating the redirect message to a large number of access units in the system, thereby seeking to reduce future misdirected transmissions, and to optimise use of the satellite""s resources, with the minimum amount of control traffic. A satellite system is better adapted to utilise a method that is not feasible in a local area network, that is, disseminating an ICMP redirect message to a large number of sources, because the access units in a satellite system tend to be routers rather than simple hosts, and the routers devote part of their processing power and memory to routing. In addition, the resources of a satellite system are a more costly commodity than the resources of a local network, and the cost consequences of not optimising routing in a satellite system are more significant than in not optimising routing in a local area network.
Preferably, a transmission contains destination information and the gateway concludes that the transmission could follow an alternative route from the destination information and from its knowledge of the organisation of the network.
Further preferably, the system comprises the Internet and the transmission comprises a datagram including destination information and data.
A multicast address may be used to carry the alternative route message, so that access units may identify it and possibly discard it when overloaded with traffic.
The up-link may be a common up-link for all of the access units.
According to a second aspect of the present invention there is provided a gateway in a satellite communications system comprising a plurality of access units communicating with a satellite on an up-link and a broad downlink and a plurality of gateways to and from a network, each gateway receiving from the satellite transmissions communicated thereto by the access units and intended for destinations in the network, at least one gateway having means which, in response to receiving a transmission which it concludes should follow an alternative route via one of the other gateways, sends an alternative route message to the satellite for forwarding using the broadcast downlink to a plurality of the access units in the system.
According to a third aspect of the present invention there is provided an access unit in a satellite communications system comprising a plurality of access units communicating with a satellite on an up-link and a broadcast downlink and a plurality of gateways to and from a network, each access unit sending to the satellite transmissions intended for destinations in the network, which access units have means for an alternative route message forwarded by the satellite from a gateways to all access units on the broadcast downlink and means for routing transmissions according to the alternative route message.